Method for the pasteurization of egg products using radio waves

ABSTRACT

A method of, and apparatus for, pasteurizing flowable egg products utilizing high frequency radio waves to heat the egg products being pasteurized.

FIELD OF THE INVENTION

The present invention relates to a method of pasteurizing flowable eggproducts utilizing high frequency radio waves.

BACKGROUND OF THE INVENTION

Large quantities of liquid egg products are pasteurized each year toproduce products sold in refrigerated form with limited shelf life. Seegenerally, Egg Pasteurization Manual (USDA Agricultural Research Service1969). More recently, the provision of techniques for ultrapasteurizingliquid egg products has led to the marketing of liquid egg productswhich have extended shelf lives under refrigerated conditions.

Methods of ultrapasteurizing liquid whole egg products using indirectheat, such as contacting the liquid whole egg product to a heatedsurface, or direct heat, such as contact to or the injection of steam,are known. See U.S. Pat. Nos. 4,808,425 to Swartzel et al.; 5,105,724 toSwartzel and Ball; and 4,957,760 to Swartzel et al.

U.S. Pat. No. 4,091,119 to Bach discloses a method and apparatus foruniform heating of a foodstuff for preservation using high-frequencyelectromagnetic fields. The foodstuff (in particular, a jam, marmaladeor jelly) is located in a container open at its top, and two fields, onehigh frequency and another ultra-high frequency, are directed asdifferent portions of the foodstuff container. Use of a high frequencyfield of between 1-300 MHz, used in conjunction with an ultra-highfrequency field of between 300 MHz and 5 GHz, is disclosed.

U.S. Pat. No. 4,853,238 to Huang discloses a method for treating liquidegg white or liquid whole egg in order to extend the shelf-life of theliquid egg material. The liquid egg material is subjected to microwaveenergy while it flows in a conduit for a time sufficient to inactivateor destroy a sufficient amount of microorganisms in the material withoutsignificantly reducing the egg protein functionality of the material.

U.S. Pat. No. 4,975,246 to Charm discloses a method and apparatus forpasteurizing and/or sterilizing heat-sensitive material usinghigh-temperature, short-time heating to substantially destroy selectedmicroorganisms without substantially affecting other desirableproperties of the material. Heat-sensitive material, such as liquid eggproduct, is continually passed through a flow path, such as plastictubing, subjected to microwave radiation to heat the material to a hightemperature for a short period, and then rapidly cooled.

SUMMARY OF THE INVENTION

Disclosed is a method of pasteurizing a flowable egg product in acontinuous flow apparatus. The method comprises the steps of: (a)passing a flowable egg product continuously through a conduittransparent to high frequency radio waves; (b) heating the flowable eggproduct to a predetermined temperature by subjecting it to highfrequency radio waves; and then (c) holding the flowable egg product atsaid predetermined temperature for a predetermined time; wherein thetotal thermal treatment of the product is sufficient to pasteurize theproduct. Preferably, the total thermal treatment is sufficient toachieve a five, six, seven or even nine log cycle reduction in Listeriamonocytogenes without significantly reducing the egg functionality.

An apparatus for pasteurizing a flowable egg product is also disclosed.The apparatus comprises: (a) an egg product holding tank; (b) an eggproduct line connected to the holding tank; (c) pumping means connectedto the egg product line for establishing an egg product stream in theegg product line; (d) a radiolucent conduit interposed in the eggproduct line; (e) a generator capable of generating high frequency radiowaves and placed so that radio waves generated pass through theradiolucent conduit to heat the egg product stream; (f) a holdingsection in the egg product line after the radiolucent conduit.

Radio frequency energy has been reported as being used to scramble eggs(See, e.g., "Magna-Tube" heat processing equipment sales brochure,Proctor Strayfield, Horsham, Pa.), but has not heretofore been suggestedfor use in pasteurizing egg. Advantages of high frequency radio wavesfor egg pasteurization include, among other things, the uniformity ofheating provided thereby, the avoidance of adding water to the productsuch as by steam injection, and the avoidance of fouling of heatedsurfaces.

The foregoing and other objects and aspects of the present invention areexplained in detail in the specification set forth below.

BRIEF DESCRIPTION OF THE DRAWINGS

The FIGURE schematically illustrates an apparatus for carrying out thepresent invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Examples of pasteurized liquid whole egg products which may be producedby the method of the present invention include whole egg, fortifiedwhole egg (whole egg with added yolk), salt whole egg (e.g., salt 10%),sugar whole egg (e.g., sugar 10%), blends of whole egg with syrupsolids, syrups, dextrose and dextrins and/or gums and thickening agents,blends of whole eggs with less than 1% sugar and/or salt, scrambled eggmixes (for example, a mix of about 51% egg solids, 30% skim milk solids,15% vegetable oil and 1.5% salt), reduced cholesterol egg products andblends thereof, custard blends, and the like, that is productscontaining at least about 10% egg solids. Products which are extremelysensitive to thermal processing and which are particularly suitable forultrapasteurization by the present invention include, for example,liquid whole egg and blends thereof (less than 2% added non-eggingredients), fortified whole egg and blends thereof (24-38% egg solids,2-12% added non-egg ingredients), liquid salt whole egg, liquid sugarwhole egg, and other liquid whole egg blends which are 24-38% egg solidsand 12% or less of added non-egg ingredients. Terms used herein havetheir standard meaning in accordance with industry and regulatory usage.See, e.g., 7 C.F.R. §59.570(b) (1985).

As used herein, ultrapasteurization means pasteurization which resultsin pasteurized products with shelf lives under refrigerated conditions(e.g., 4° Centigrade or less, but above freezing) greater than thatobtainable using previously known pasteurization methods. See, e.g., K.Swartzel et al., U.S. Pat. No. 4,808,425 (the disclosure of which isintended by applicants to be incorporated herein in its entirety). Thusultrapasteurization methods produce extended shelf life products, e.g.,products having shelf lives more than 10 days, and preferably more than14 days, and typically 4 to 6 weeks and up to 36 weeks or more.

Methods and apparatus of the present invention are also suitable forpasteurizing shell eggs (i.e., eggs contained in the shell) and forpasteurizing egg products in individual consumer packages, such asrestructured reduced-cholesterol eggs. Shell eggs and packaged liquideggs or egg products can be placed in a liquid media so that they areflowable, i.e., able to flow through the pasteurizing apparatus of thepresent invention. Additionally, egg components (such asde-cholesterolized egg yolk) may be encapsulated in an edible material.The resulting capsules can then be carried in a liquid medium(preferably another egg component such as egg white) for pasteurizationusing the method and apparatus of the present invention. As used herein"flowable" means that the product to be pasteurized is capable offlowing through the pasteurizing apparatus of the present invention,either without the addition of a carrier or in a liquid carrier medium.Examples of flowable products include liquid egg products andparticulate products carried in a liquid medium.

It is recommended that the equivalent point method for evaluatingthermal treatments be applied in practicing the present invention. Thismethod describes the total thermal treatment received by a product incontinuous flow equipment. Procedures for using the equivalent pointmethod for analyzing the thermal effects on products during continuousflow heating have been previously outlined (Swartzel (1982), J. FoodScience, 47, 1886, and Swartzel (1986) J. Agric. Food Chem. 34, 397;U.S. Pat. No. 4,808,425 to Swartzel et al.) and are known to thoseskilled in the art.

The present invention utilizes that portion of the electromagneticspectrum associated with radio reception (i.e., radio waves having afrequency of from about 500 Kilohertz (KHz) to about 110 Megahertz(MHz); or radio waves with wavelengths from about 1 meter to 10⁴meters). In particular, the present invention uses high frequency radiowaves. As used herein, high frequency radio waves (HFRW) meanselectromagnetic radiation having a frequency of from about 15 MHz toabout 150 MHz, or wavelengths of from about 2 meters to about 20 meters.Alternating currents generate electromagnetic waves of a desiredfrequency and wavelength, which travel at a speed characteristic of themedia in which they are traveling. The wavelength (γ) of a particularwave in a given egg product is determined from a knowledge of thefrequency f, which remains constant (a function of the generator), andv, which depends on the velocity of the wave in the product.

The present invention utilizes high frequency radio waves to produceheat within the products being treated, causing microbial destructionwithout loss of product functionality, and yielding reduced oreliminated product deposition on the surfaces in contact with theproduct. It is not yet established whether microbial inactivation usingelectromagnetic waves is due to thermal effects only, as in conventionalheating processes, or includes athermal effects resulting from unknowninteractions between biochemical constituents of microbes and anelectromagnetic field. See, e.g., Adey, Biological effects of radiofrequency electromagnetic radiation, In: Electromagnetic Interactionwith Biological Systems, Lin (Ed.)Plenum Press, New York, pp. 109-140(1989). However, microbial destruction with electromagnetic wavesproducing heat generally yields results similar to that usingconventional heat only. See, e.g. Goldblith, Principles and applicationof radio-frequency energy to food preservation with particular referenceto concentration and dehydration, In: Freeze Drying and Advanced FoodTechnology, Goldblith, Rey & Rothmayr (Eds.), Academic Press, New York,pp. 691-714 (1975).

In the present invention, high frequency radio waves (HFRW) areconverted to heat as they interact with the egg products. Absorption ofelectromagnetic energy increases the kinetic energy of the molecules ofthe absorbing medium, and increases the temperature of the absorbingmedium. As heat is generated within the product being heated, contactwith heated surfaces acting as heat transfer surfaces is not required.Thus fouling or burning of egg at heated surfaces is reduced oreliminated when using HFRW pasteurization techniques, allowing extendedprocess run-times. Thus the present invention will yield greaterefficiency by achieving higher through-put of product before cleaning ofequipment is required, while producing product with good functionalcharacteristics.

Most pasteurization processes using indirect heat exchangers aredesigned to maximize turbulent, high-shear flow in order to achieveefficient heat transfer throughout the egg product. In HFRW heating,particulate matter heats at the same rate as liquids, allowing thepasteurization apparatus to be designed with less concern about the flowcharacteristics of the egg product. Shear stress on the proteins can bereduced, and the need to make highly homogeneous liquids from eggs canbe eliminated. Thus low shear pumps may be used in practicing thepresent invention so that the identity of the yolks and egg whites isnot lost.

HFRW heating is distinguished from higher energy microwave heating inthat it is a process that is generally easier to control as the input ofenergy, and hence heating, is not as rapid. Microwaves are that portionof the electromagnetic energy spectrum with frequencies of from 10³ to10⁵ MHz. Additionally, larger capacity heating sections can beconstructed for use with HFRW (i.e., the cross-sectional area of a HFRWheater can be larger than that used in microwave heating processes) asthe penetration depth of HFRW energy is greater than that of microwaveenergy. It is a well-known phenomena that as frequency increases,penetration of radio wave energy into matter decreases.

HFRW heating is distinguished from ohmic heating in that the heaterdesign and controls are not dependent on the specific conductivity ofthe material being heated. For example, whole egg and scrambled eggmixes (whole egg with milk and salt) have sufficiently differentelectrical conductance that it is extremely difficult to heat them withthe same ohmic heater, while a HFRW heater should be able to heat eachproduct equally efficiently. HFRW heating does not create free radicalsand the resulting deterioration of flavor as is found when high energyionizing radiation is used to pasteurize egg products.

Any means for generating electromagnetic waves of the desiredfrequencies may be used to carry out the present invention. Anycommercial or industrial generator capable of producing high frequencyradio waves may be used. Generators may be added in parallel or inseries to increase production or temperature. Generators may beharmonically suppressed or otherwise structured to meet standards forelectromagnetic or radio frequency emissions. In a general embodiment ofheating using HFRW, the product to be heated is situated between twoelectrode plates which are charged alternately positive and negative.The plates are connected to the radio frequency generator.

The conduit carrying the egg product adjacent to the HFRW generator ismanufactured of material that is radiolucent, that is, is permeable toradio waves. As used herein, "radiolucent" means only that the materialis essentially transparent to radio waves of the frequency used in themethod of the present invention; while the material may be permeable toelectromagnetic waves of other frequencies, this is not required. Theconduits are preferably manufactured from a suitable synthetic plasticsmaterial. Examples of suitable materials include polytetrafluoroethylene(e.g., the product marketed as TEFLON™ or HOSTAFLON™), and polycarbonateresins such as LEXAN™.

Any means for establishing a continuous stream of egg product may beused to carry out the present invention. Pumps used to establish thestream may or may not be positive displacement pumps, though positivedisplacement pumps (timing pumps) are generally needed to preciselydefine the holding time of a product stream in a holding section.Positive displacement pumps may be used in combination with otherpumping means, such as centrifugal pumps.

Ingredients may be added to or removed from the product stream in thepresent method at any location to provide the various egg products givenabove. The removal of cholesterol is discussed specifically below. Minoringredients, such as salt, citric acid or phosphate to stabilize color,gums, oils, colors or flavors, may be added to the product stream, andmay be added at any suitable time or location in the pasteurizingprocess.

A cholesterol removal step may optionally be included in the process ofthe present invention. The particular cholesterol removal procedureemployed is not critical. Exemplary processes include those disclosed inA. Bayol et al., European Patent Application No. 89400175.9; U.S. Pat.No. 4,714,556; U.S. Pat. No. 4,361,484; U.S. Pat. No. 4,234,619; U.S.Pat. No. 3,882,034, and U.S. Pat. No. 3,607,304 (applicants specificallyintend the disclosures of all U.S. patents cited herein to beincorporated herein in their entirety).

Using the method and apparatus of the present invention, it is possibleto treat egg products from temperatures below 40° F. (but abovefreezing) up to temperatures above 160° F., but below cookingtemperatures. The product is then held at the final temperature for aperiod of time adequate to destroy harmful and spoilage bacteria, asdiscussed below.

An optional preheating step may be employed prior to HFRW pasteurizationto preheat the egg product to a temperature between 140° F. and 155° F.Preheating systems may be composed of, but are not limited to,conventional heating systems such as plate, swept, tube heat exchangers,microwave heating systems, ohmic systems, steam injection, hot waterinjection, hot fluid food injection, etc.

The total thermal treatment received by the egg product during theprocess must be sufficient to cause at least a nine log cycle reductionof Salmonella bacteria in the product (i.e., sufficient to pasteurizethe product). Proper thermal treatment is typically insured bypresetting the holding times. The term "holding time", as used herein,has its ordinary meaning as used in the industry, and all log cyclereductions referred to herein concern processed product as compared toraw or unprocessed product. Preferably, the thermal treatment issufficient to produce a product having a shelf life of about four weeksto about thirty-six weeks under refrigerated conditions, and morepreferably a product having a shelf life of about eight weeks to aboutthirty-six weeks under refrigerated conditions. The term "refrigerated,"as used herein, means stored at or below a temperature of 4° Centigradebut above freezing.

Most preferably the holding time for the egg product is sufficient tocause a five, six, seven or even nine log cycle reduction in Listeriamonocytogenes (Listeria) in the product. Listeria is found in a widerange of animals (including man), on plants, and in soil. SeeMicrobiology, 799-800 (B. Davis et al., Eds., 3d Ed. 1980) (Harper &Row). A treatment capable of causing a nine log cycle reduction inListeria is preferred because the wide-spread nature of Listeria makesit difficult to exclude from a pasteurizing plant, because of thepathogenic nature of this microorganism (see Id.) and because Listeriaappears capable of growing to large quantities in egg under refrigeratedconditions (the conditions contemplated for the products of the presentprocess before preparation and consumption).

Following pasteurization the product can then be cooled usingconventional cooling systems such as, but not limited to, plate heatexchangers, swept surface heat exchangers, liquid nitrogen injection,CO₂ gas injection, or injection of other inert gases.

Elements of the apparatus are interconnected by a product line formed ofany conventional sanitary material, such as stainless steel tubing.

Raw product may be provided by conventional shell egg breakingapparatus, such as a Seymore breaker (not shown). The breaking apparatusmay be in the same plant as the apparatus of the present invention, maybe operatively connected to the apparatus of the present invention, ormay be located in a separate plant.

To obtain a product with reduced quantities of microorganisms, thepasteurizing apparatus should be sterilized before the egg product ispassed therethrough. Sterilizing is preferably accomplished by passinghot water under pressure through the pasteurizing apparatus, as is knownin the art, so that hot water is contacted to those surfaces whichcontact the product at a temperature and pressure and for a timesufficient to sterilize these surfaces.

The product, after pasteurization, should be aseptically packaged.Aseptically packaged means packaged to the exclusion of microorganismsother than those carried by the egg product itself. Equipment suitablefor aseptically packaging liquid egg products is commercially available.Also useful in carrying out this step is equipment which packages theproduct to the substantial exclusion of microorganisms, known in theindustry as "clean fillers," but the greater exclusion of microorganismsprovided by aseptic fillers makes aseptic fillers preferable,particularly in view of the ability of Listeria to grow underrefrigerated conditions, as discussed above.

A homogenization step may optionally be included after the holding step.The term "homogenization" as used herein, means to subject a product tophysical forces to reduce particle size. Such procedures are known inthe art, and may be carried out on different types of equipment. It ispreferable to carry out this homogenizing step with homogenizingequipment at total pressures of from about 500 p.s.i. to about 3,000p.s.i.

An apparatus for carrying out the method of the present invention isshown schematically in the FIGURE. Arrowheads indicate the direction offlow of the product. This apparatus is readily assembled fromcommercially available parts. The apparatus of the FIGURE comprises araw product holding tank (10) and a first pump (11) to pump the rawliquid product out of the holding tank and into the radiolucentpasteurization conduit (13). A preheater (12) may optionally be locatedintermediate the first pump (11) and the radiolucent conduit (13). Agenerator capable of generating HFRW (14) is located adjacent to theradiolucent conduit (13), such that high frequency radio waves generatedpass through the radiolucent pasteurization conduit. A positivedisplacement pump (15) is positioned in the product line after theradiolucent pasteurization conduit, and a check valve (16) is positionedin the product line after the positive displacement pump and before theholding section (20). A flow diversion valve (21) in the product linefollowing the holding section is provided for shunting improperlytreated product to a holding tank (not shown) via a diversion line (22).The length of the holding section (20) is chosen to provide the desiredholding time to the egg product carried therein. Optionally, ahomogenizer (23) such as the Cherry Burrell Model HD-6 aseptichomogenizer follows the flow diversion valve. A cooler (24) such as theCherry-Burrell Model 558X8 No-Bac SPIRATHERM™ tube-in-shell coolerfollows the flow diversion valve and the (optional) homogenizer. Anaseptic filler (25) such as the TETRA PAK™ Model AB-3-250 receivespasteurized egg product and aseptically packages the whole egg productstream carried therein in individual cartons.

It will be readily apparent to those of ordinary skill in the art thatmodifications of the pumping system described above will be required forpasteurizing non-liquid products, such as eggs in the shell, orpre-packaged egg products. Such products also may not require asepticpackaging following pasteurization.

In use for carrying out a method of the present invention forpasteurizing a liquid whole egg product, the apparatus of the FIGUREmay, for example, be set so that HFRW having a wavelength of about 10meters are generated and heat the liquid whole egg product in theradiolucent pasteurization conduit to about 64°-70° Centigrade. Theholding section (20) is a length suitable to provide a holding time forthe liquid egg product of about 2 to 5 minutes to thereby produce apasteurized liquid egg product. To produce a product with greater killsof Listeria, the holding section is lengthened to give a holding time ofup to 9 minutes for some products.

In another embodiment of the present invention, a preheater (12) such asa Cherry-Burrell Model E SUPERPLATE™ preheater, preheats the egg productto a temperature between 135° F. and 155° F. prior to the egg productbeing pumped into the radiolucent conduit, where the final portion ofheating is achieved by HFRW.

Those skilled in the art will appreciate that the present invention canbe combined with the features of other egg pasteurization techniques,such as described in U.S. Pat. Nos. 4,808,425, 5,105,724, and 4,957,760(the disclosures of which are intended by applicants to be incorporatedherein in their entirety), and that these techniques can be resorted tofor further guidance in practicing the present invention.

Those skilled in the art will appreciate that minor variations can bemade in the apparatus and procedures described herein without departingfrom the spirit of the present invention. Thus the invention is definedby the following claims, with equivalents of the claims to be includedtherein.

That which is claimed is:
 1. A method of pasteurizing a flowable eggproduct while passing the product as a continuous stream through apasteurizing apparatus, comprising:passing said flowable egg productcontinuously through a conduit transparent to high frequency radiowaves, wherein said flowable egg product comprises shell eggs in aliquid medium; and heating said flowable egg product in said conduit toa predetermined temperature by subjecting said egg product to highfrequency radio waves; and then holding said flowable egg product atsaid predetermined temperature for a predetermined time to provide atotal thermal treatment to said egg product; wherein the total thermaltreatment of said product is sufficient to pasteurize said egg productwithout coagulation thereof, and wherein said high frequency radio wavesare in the frequency range from about 15 MHz to 150 MHz.
 2. A method ofpasteurizing a flowable egg product while passing the product as acontinuous stream through a pasteurizing apparatus, comprising:.passingsaid flowable egg product continuously through a conduit transparent tohigh frequency radio waves, wherein said flowable egg product comprisespre-packaged egg products in a liquid medium; and heating said flowableegg product in said conduit to a predetermined temperature by subjectingsaid egg product to high frequency radio waves; and then holding saidflowable egg product at said predetermined temperature for apredetermined time to provide a total thermal treatment to said eggproduct; wherein the total thermal treatment of said product issufficient to pasteurize said egg product without coagulation thereof,and wherein said high frequency radio waves are in the frequency rangefrom about 15 MHz to 150 MHz.